Major challenges in electro-statically actuated optical micro-electro-mechanical (MEMS) devices is to achieve a relatively high angle of rotation and to lower the actuation voltage, especially in the switching axis, i.e. the “piano” or Y axis. Conventional biaxial MEMS devices, such as those disclosed in U.S. Pat. Nos. 6,934,439 issued Aug. 23, 2005 in the name of Miller et al, and U.S. Pat. No. 7,095,546 issued Aug. 22, 2006 in the name of Mala et al include two sets of parallel plate electro-static electrodes for both tilt (Y-axis) and roll (X-axis) movements requiring complicated electrode and supply configurations, such as the ones disclosed in U.S. Pat. Nos. 6,968,101 issued Nov. 22, 2005, and U.S. Pat. No. 7,010,188 issued Mar. 7, 2006 both in the name of Miller et al. providing limited tilt angle (Y axis) range and control.
Parallel plates (PP) electro-static electrodes suffer from pull-in instability, which limits useable angular range; accordingly, parallel plate electrodes for both piano tilt and roll does not provide sufficient range for next generation devices.
A vertical comb drive is a type of MEMS actuator capable of relatively high actuator power using electrostatic principals of operation, and can be fabricated using standard materials and scalable processes developed in the semiconductor industry. Vertical comb drives can be advantageously used to control high-speed, high-resolution micro-mirrors in a variety of optical applications including optical scanning, optical switching, free-space optical communications, optical phased arrays, optical filters, external cavity lasers, adaptive optics and other applications.
The actuation principle of a typical vertical comb drive is electrostatic, wherein a potential difference is applied between two comb structures, a movable comb (or a rotor), and a stationary comb (or a stator). When a voltage is applied between them, a torque is developed from the electrostatic filed causing the movable comb to rotate about supporting hinges toward the stationary comb until the electrostatic torque is balanced by the restoring mechanical torque of the hinge springs. Different types of vertical comb drive devices are described in further detail, for example, in U.S. Pat. No. 6,612,029 issued to Behin et al, which is incorporated herein by reference.
Conventional vertical comb drives are relatively efficient compared to parallel plate electro-static electrode actuators, and may be designed to avoid the pull-in phenomenon in the actuation direction associated with parallel plate electrodes. However, a major challenge with vertical comb drives is the sub-micron comb finger alignment accuracy that is required for the stability of the actuator.
One type of comb actuator is a staggered vertical comb (SVC) drive in which the rotor and stator combs are fabricated in different layers. A typical prior art process flow involves creating the moving comb assembly by etching one silicon-on-insulator (SOI) wafer, and creating the stationary comb assembly by etching another SOI wafer, and then assembling the two etched wafers together to form the vertical comb drive. Different versions of such process are described in U.S. Pat. Nos. 6,925,710 and 7,079,299. However, stringent alignment requirements between the two wafers from which the two comb assemblies are formed can considerably complicate the device processing and negatively affect the device yield. A self-aligned mask process has been developed to overcome this issue disclosed in U.S. patent application Ser. No. 11/733,821 filed Apr. 11, 2007 in the name of Moffat et al, which is incorporated herein by reference, although such a self-aligned SVC process is relatively complex.
Another type of comb actuator is an angular vertical comb (AVC), in which both rotor and stator combs are fabricated in the same layer, and the rotor combs are pre-tilted or biased with respect to the stator combs later, preferably during the device release process. The major advantage of AVC is that the combs are automatically self-aligned as they are fabricated in the same device layer.
Unfortunately, in a two-dimensional gymbol arrangement, mirror tilt about the Y-axis affects the X-electrode geometry; therefore, if vertical comb drives were provided for both the X and Y tilting, the narrow finger spacing (as would be the case say in a self-aligned comb process), in the Y-axis tilt would lead to X stator/rotor interference that may cause lateral snap.
An object of the present invention is to provide a micro-mirror pivotable about two orthogonal axes (X and Y) with an actuator array structure using combs in the switching axis (Y) to obtain a relatively large tilt angle and/or to reduce the required voltage, while rotation about the X axis is achieved by a relatively easily manufactured parallel plate electro-static actuator, wherein the combs are decoupled from mirror roll as they are arranged internal to the X-axis hinge.